Printhead with close-packed configuration of alternating sized drop ejectors and method of firing such drop ejectors

ABSTRACT

A printhead uses large and small drop ejectors to achieve efficient gray scale printing. The printhead is arranged with a close packed configuration of alternating large and small nozzles positioned to maximize coverage while minimizing the volume of ejected ink. The printhead may be operated in a single pass mode or dual pass mode. In the single pass mode, complete coverage is effected by rippling through the odd numbered jets first and then rippling through the even numbered jets. The position of the small spots from the even numbered jets can be adjusted to maximize coverage and counteract offset between nozzle centers. Printheads with different size nozzles can also be operated by a staggered firing method using dual passes to offset spots in the scan direction by shifting the printhead between passes or alternating between groups of large and small nozzles. Further improvements to image quality can be realized by shifting the spots in the direction perpendicular to the scanning direction by tilting the printhead or offsetting the nozzles with respect to the ink channels on the printhead.

RELATED APPLICATION

[0001] This application is related to U.S. Ser. No. ______, filedsimultaneously herewith.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates generally to a liquid ink printingapparatus and a method for gray scale printing. More particularly, theinvention relates to an ink jet printhead having different size dropejectors.

[0004] 2. Description of Related Art

[0005] Liquid ink printers of the type frequently referred to ascontinuous stream or as drop-on-demand, such as piezoelectric, acoustic,phase change wax-based or thermal, have at least one printhead fromwhich droplets of ink are ejected towards a recording sheet. Within theprinthead, the ink is contained in a plurality of channels. Power pulsescause the droplets of ink to be expelled as required from orifices ornozzles at the end of the channels.

[0006] In a thermal ink-jet printer, the power pulse is usually producedby a heater transducer or resistor, typically associated with one of thechannels. Each resistor is individually addressable to heat and vaporizeink in the channels. As voltage is applied across a selected resistor, avapor bubble grows in the associated channel and initially bulges fromthe channel orifice followed by collapse of the bubble. The ink withinthe channel then retracts and separates from the bulging ink therebyforming a droplet moving in a direction away from the channel orificeand towards the recording medium whereupon hitting the recording mediuma dot or spot of ink is deposited. The channel is then refilled bycapillary action, which, in turn, draws ink from a supply container ofliquid ink.

[0007] An ink jet printhead can include one or more thermal ink jetprinthead dies having an individual heater die and an individual channeldie. The channel die includes an array of fluidic channels which bringink into contact with the resistive heaters which are correspondinglyarranged on the heater die. In addition, the die may also haveintegrated addressing electronics and driver transistors. Fabricationyields of die assemblies at a resolution on the order of 300-600channels per inch is such that the number of channels per die ispreferably in the range of 50-500 under current technology capabilities.Since the array of channels in a single die assembly is not sufficientto cover the length of a page, the printhead is either scanned acrossthe page with a paper advance between scans or multiple die assembliesare butted together to produce a page width printbar. Because thermalink jet nozzles typically produce spots or dots of a single size, highquality printing requires the fluidic channels and corresponding heatersto be fabricated at a high resolution on the order of 400-600 channelsper inch.

[0008] The ink jet printhead may be incorporated into either a carriagetype printer, a partial width array type printer, or a page-width typeprinter. The carriage type printer typically has a relatively smallprinthead containing the ink channels and nozzles. The printhead can besealingly attached to a disposable ink supply cartridge. The combinedprinthead and cartridge assembly is attached to a carriage which isreciprocated to print one swath of information (equal to the length of acolumn of nozzles), at a time, on a stationary recording medium, such aspaper or a transparency. After the swath is printed, the paper isstepped a distance equal to the height of the printed swath or a portionthereof, so that the next printed swath is contiguous or overlappingtherewith. This procedure is repeated until the entire page is printed.In contrast, the page width printer includes a stationary printheadhaving a length sufficient to print across the width or length of asheet of recording medium at a time. The recording medium is continuallymoved past the page width printhead in a direction substantially normalto the printhead length and at a constant or varying speed during theprinting process. A page width ink-jet printer is described, forinstance, in U.S. Pat. No. 5,192,959.

[0009] Printers typically print information received from an imageoutput device such as a personal computer. Typically, this receivedinformation is in the form of a raster scan image such as a full pagebitmap or in the form of an image written in a page descriptionlanguage. The raster scan image includes a series of scan linesconsisting of bits representing pixel information in which each scanline contains information sufficient to print a single line ofinformation across a page in a linear fashion. Printers can print bitmapinformation as received or can print an image written in the pagedescription language once converted to a bitmap consisting of pixelinformation.

[0010] In a printer having a printhead with equally spaced nozzles, eachof the same size nozzles producing ink spots of the same size, thepixels are placed on a square first grid having a size S, where S isgenerally the spacing between the marking transducers or channels on theprinthead as illustrated in a sample printing pattern of FIG. 2. Thenozzles 60 (schematically represented as triangles) traverse across arecording medium in the scan direction X as illustrated. The nozzles,which are spaced from one another a specified distance d, also known asthe pitch, deposit ink spots or drops on pixel centers 62 on the gridhaving the grid spacing S in a direction perpendicular to the scanneddirection, which is of course dependent on the spacing d. Typically, thenozzles and printing conditions are designed to produce spot diametersof approximately 1.414 (the square root of 2) times the grid spacing S.This allows complete filling of space, by letting diagonally adjacentpixels touch. A disadvantage of this printing scheme is that jaggednessmay be objectionable at line edges, particularly for lines or curves atsmall angles to the scan direction as illustrated in FIG. 2. A firstellipse 64 located outside a second ellipse 66 in FIG. 2, indicate atwhat portions of the printed image the jaggedness would be mostobjectionable. In addition, print quality can be determined by 1) howmuch white space remains within the ring defined by the first and secondellipses, 2) how far the spots extend outside either the first or secondellipse, and 3) the amount of ink deposited on the recording medium.

[0011] One method of improving the line edge quality is to extend theaddressability of the carriage to thereby allow dot placement atintermediate positions in the grid in the scanned direction. It is alsopossible to improve line edge quality by increasing the resolution.This, however, increases the complexity and cost of fabrication andtypically slows down printing because of the additional number of spotsto be printed.

[0012] The printheads and printing methods discussed above, andillustrated in FIG. 2 for example, provide for the printing of ink jetimages having sufficient quality, especially when the resolution isincreased upwards to 600 channels per inch. These printheads andmethods, however, do not always provide images having the desiredquality especially when considering gray scale levels, ink saving printmodes, and printing throughput.

[0013] A majority of thermal ink jet printers produce spots or drops ofink all having the same diameter, within approximately about 10 percent,and are therefore not capable of gray scale printing. Drop volume orspot size is determined by many factors, including the heater transducerarea, the cross sectional area of the ink ejecting channel or nozzle,the pulsing conditions necessary to create an ink droplet and thephysical properties of the ink itself, such as the ink temperature.Although spot diameter changes of approximately ±10 percent are possibleby changing pulsing conditions or ink temperature during printing, thegiven spot size is nominally constant to the extent that deliberate spotsize variations cannot span a large enough range to be useful in grayscale printing.

[0014] Another method of improving printing quality, especially grayscale printing quality is to use groups of different size nozzles, asdisclosed in U.S. Pat. No. 5,745,131 to Kneezel et al., which is herebyincorporated by reference into this disclosure. FIG. 3 illustratesprinting according to U.S. Pat. No. 5,745,131 wherein a pattern isprinted with a printhead having a first plurality of orifices 67 and asecond plurality of orifices 68, producing spot diameters of 1.4 S and1.0 S respectively. The spacing between nozzles of the first pluralityof orifices 67 is again the distance d and the spacing betweenindividual nozzles of the second plurality of orifices 68 is also thespacing d. The printing grid illustrated in FIG. 3 has a spacing of Sbetween the pixel centers. The ink jet printer fires the individualnozzles of each plurality of orifices so that the ink drops land on thegrid points in the scan direction. A somewhat better fill is achievedthan previously illustrated in FIG. 2, at least in terms of the amountof ink used. Within the first ellipse 64 and the second ellipse 66,there are thirty-eight pixels of the large ink drops and sixteen pixelsof the smaller. ink drops which yields a more extensive coverage of inkwithin the first ellipse 64 and the second ellipse 66, even though thetotal amount of ink used is actually less than in FIG. 2. Since thenumber of nozzles within each of the first plurality of nozzles 67 andthe second plurality of nozzles 68 are equivalent, the paper is advancedhalf the printhead length to achieve proper fill.

[0015] Various other methods and apparatus for gray scale printing withthermal ink jet printers and other ink jet printers include changing theink drop size by either varying the driving signals to the transducerwhich generates the ink droplet or by creating a printhead which has anumber of different sized ink ejecting orifices for creating gray scaleimages.

[0016] For example, U.S. Pat. No. 5,412,410 to Rezanka, discloses aprinthead having different sized nozzles, which are alternated with eachother according to size. As shown in FIG. 4, printhead 30 has large sizenozzles 32 alternated with relatively small size nozzles 34 across thelinear array. Each nozzle is spaced a distance S on center, with thelarge and small nozzles spaced apart by 2 S, respectively. While grayscale printing can be effected by this arrangement, a large volume ofink is used and printing throughput or speed can be slow.

SUMMARY OF THE INVENTION

[0017] This invention addresses the above problems by providing aprinthead with different size nozzles to effectively and efficientlyfill spaces between pixels.

[0018] The printhead according to this invention includes a plurality ofdrop ejectors, including a first set of drop ejectors having a firstsize and a second set of drop ejectors having a second size. The firstset of drop ejectors and the second set of drop ejectors are arranged ina single linear array with adjacent drop ejectors having different sizesto form a pattern of alternating first and second size drop ejectors.

[0019] Each drop ejector in the first set of drop ejectors has an axialcenter point and each drop ejector in the second set of drop ejectorshas an axial center point, which is diagonally offset with respect tothe center points of the first set of drop ejectors.

[0020] To minimize ink usage, the drop ejectors having a same width arespaced ROM each other a distance S, wherein the spots formed by the dropejectors have a diameter less than S42.

[0021] Preferably, in the preferred embodiment, the printhead isdisposed in a printing device including a movable carriage that supportsthe printhead for movement in a scanning direction and a controllerconnected to the carriage to control movement of the printhead and tothe actuators to control actuator of the drop ejectors.

[0022] The printhead with alternating width drop ejectors ejects spotsformed by the large drop ejectors with a diameter D that equals aproduct of spacing between same size drop ejectors S and a constant a(where 1.0<a <{square root}2), according to: D=aS. The point ofintersection between two adjacent large spots and a small spot occurs adistance x measured from a vertical center line extending between theadjacent large spots, according to: x=0.5 S(a2−1)0.5. By this, efficientcoverage with minimum ink can be determined.

[0023] According to this invention, the method of firing ink dropletsfrom different width ejectors arranged in an alternating pattern in alinear array on a printhead, including odd numbered ejectors having afirst width and even numbered ejectors having a second width differentfrom the first width, comprises the steps of consecutively firing oddnumbered ejectors to eject ink spots, consecutively firing even numberedejectors to eject ink spots, and controlling firing of the even numberedejectors to eject even fired ink spots in spaces between the odd firedink spots. Firing even numbered ejectors ejects spots having a diametersmaller than spots ejected from the odd numbered ejectors.

[0024] Preferably, the steps of consecutively firing odd numberedejectors and consecutively firing even numbered ejectors occurs in asingle printing pass. The step of consecutively firing the even numberedejectors can occur after moving the printhead a distance equal ton+{fraction (1/2)} pixels in the scanning direction where n is aninteger. Controlling the firing of the even numbered ejectors caninclude delaying or advancing the printhead in the scanning directionrelative to a position for firing the odd numbered ejectors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other objects, advantages and further features of this inventionwill be apparent from the following, especially when considered with theaccompanying drawings, in which:

[0026]FIG. 1 is a partial schematic perspective view of an ink jetprinter incorporating this invention;

[0027]FIG. 2 illustrates the locations of ink spots in a test patterndeposited by a printhead having ink ejecting nozzles of the same size;

[0028]FIG. 3 illustrates the locations of ink spots in a test patterndeposited by a printhead having ink ejecting nozzles of two differentsizes;

[0029]FIG. 4 is a front view of a printhead having nozzles of differentsizes disposed in an alternating pattern;

[0030]FIG. 5 is schematic block diagram of a control system inaccordance with this invention;

[0031]FIG. 6 is a schematic diagram of the alternating size nozzles inaccordance with this invention;

[0032]FIG. 7A shows a pattern of spots ejected from a printhead with thesame size nozzles;

[0033]FIG. 7B shows a pattern of spots ejected from a printhead withdifferent size nozzles in accordance with this invention;

[0034]FIG. 7C shows another pattern of spots ejected from a printheadwith different size nozzles in accordance with this invention;

[0035]FIG. 8 is a schematic diagram showing the method of determiningthe optimum spacing and overlap between spots according to thisinvention;

[0036]FIGS. 9A and 9B show a pattern of spots deposited in a first andsecond pass, respectively, according to a staggered method of firingaccording to this invention;

[0037]FIGS. 10A and 10B show another pattern of spots deposited in afirst and second pass, respectively, according to a staggered method offiring according to this invention;

[0038]FIG. 11A shows a tilted printhead usable in a method of firing inaccordance with this invention;

[0039]FIG. 11B shows the pattern of spots ejected in accordance with theprinthead of FIG. 11A;

[0040]FIG. 12A is a schematic diagram of alternating sized nozzleshaving the nozzles offset from the associated channel centers inaccordance with this invention; and

[0041]FIGS. 12B and 12C show patterns of spots deposited on a first andsecond pass, respectively, according to a method of firing of thisinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0042]FIG. 1 illustrates a partial schematic perspective view of an inkjet printer 10 having an ink jet printhead cartridge 12 mounted on acarriage 14 supported by carriage rails 16. The printhead cartridge 12includes a housing 18 containing ink for supply to a thermal ink jetprinthead 20 which selectively expels droplets of ink under control ofelectrical signals received from a controller of the printer 10 throughan electrical cable 22. The printhead 20 contains a plurality of inkchannels, which carry ink from the housing 18 to respective inkejectors, such as orifices or nozzles.

[0043] When printing, the carriage 14 reciprocates or scans back andforth along the carriage rails 16 in the directions of the arrow 24. Asthe printhead cartridge 12 reciprocates back and forth across arecording medium 26, such as a sheet of paper or transparency, dropletsof ink are expelled from selected ones of the printhead nozzles towardsthe sheet of paper 26. The ink ejecting orifices or nozzles aretypically arranged in a linear array perpendicular to the scanningdirection 24. During each pass of the carriage 14, the recording medium26 is held in a stationary position. At the end of each pass, however,the recording medium is stepped by a stepping mechanism under control ofthe printer controller in the direction of an arrow 28. For a moredetailed explanation of the printhead and printing thereby, refer toU.S. Pat. No. 4,571,599 and U.S. Pat. No. Reissue 32,572, which areincorporated herein by reference.

[0044] The carriage 14 is moved back and forth in the scanningdirections 24 by a belt 38 attached thereto. The belt 38 is driven by afirst rotatable pulley 40 and a second rotatable pulley 42. The firstrotatable pulley 40 is, in turn, driven by a reversible motor 44 undercontrol of the controller of the ink jet printer in addition to thetoothed belt/pulley system for causing the carriage to move. It is alsopossible to control the motion of the carriage by using a cable/capstan,lead screw or other mechanisms as known by those skilled in the art.

[0045] To control the movement and/or position of the carriage 14 alongthe carriage rails 16, the printer includes an encoder having an encoderstrip 46 which includes a series of fiducial marks in a pattern 48. Thepattern 48 is sensed by a sensor 50, such as a photodiode/light sourceattached to the printhead carriage 14. The sensor 50 includes a cable 52that transmits electrical signals representing the sensed fiducial marksof the pattern 48 to the printer controller.

[0046] The printer controller can be a portion of any type of knowncontrol system typically used for selectively controlling nozzlefunction based on image data. An exemplary control system suitable forthis invention is shown in FIG. 5. As seen, the printer controller orcontrol system 120 includes a clock 122 having an output connected to afirst counter 124. A second counter 126 is serially connected to thefirst counter 124. The clock 122 generates a sequence of clock pulseswhich advances the two counters serially connected together. A printercontroller 128 controls the first counter 124 and the second counter 126through separate control lines.

[0047] In addition, the control system 120 includes a RAM 130 having adata/input line 132 and a read/write input line 134 connected to thecontroller 128. The RAM 130 receives data or input information from aprinter interface which is connected to an image generating system suchas a personal computer. The RAM 130 stores image information which caninclude an entire document, a single line thereof, or a single loadingof the printhead. An output line 136 of the RAM 130 is connected to aROM 137 which contains the bitmapped patterns to be printed. An outputline 136 of the RAM 30 is connected to a ROM 137 which contains thebitmapped patterns to be printed. The stored bitmapped patterns may beeither alphanumeric characters for printing text, or might include aplurality of halftone cells each representing a different gray level.

[0048] In operation, the clock 122 generates a sequence of clock pulseswhich advances the first counter 124 which, in turn, advances the secondcounter 126. The second counter 126 generates a word over a plurality ofoutput lines 138. The word present on the plurality of output lines 138is applied to the RAM 130 to select a portion of the image to beprinted. Typically, the word appearing on the output lines 138 is anaddress of the data stored in the RAM. The data stored in the RAM couldinclude a number of from one to N, where N is equal to the number ofdifferent gray levels which can be printed.

[0049] The first counter 124 includes a plurality of output lines 140connected to the ROM 137. The counter 124 selects the particular part ofthe pattern or halftone cell to be loaded into the printhead based on anoutput 136 of the RAM 130 which is an address for the ROM 137 containingthe bitmapped pattern to be printed. Once the first counter 124 selectsthe particular portion of the bitmap pattern to be loaded, the ROM 137outputs the necessary data over a first data line 142 connected to aprinthead 20, which prints large and small spots.

[0050] The printhead 20 has different size drop ejectors or nozzleswithin a single printhead die, as shown in FIG. 6. The informationoutput to printhead 20 is loaded by a shift register (not shown)resident in the printhead. An example of such a shift register andappropriate printhead electronics for use in the present invention isdescribed in U.S. Pat. No. 5,300,968 to Hawkins, herein incorporated byreference. When the loading of the data to the printhead 20 is complete,the information is latched and the individual nozzles eject ink whilethe next row of data is being loaded into the printhead 20. It ispossible to load several rows of data for each output of the RAM 130. Inthis way, the printer controller 128 is not burdened with the task ofgenerating the specific bitmap for each density level.

[0051]FIG. 6 shows the preferred arrangement of alternating large andsmall drop ejectors, with large nozzles 70 disposed directly adjacentsmall nozzles 72 within a single array on printhead 20. In thisarrangement, the primary or large nozzles 70 are spaced apart at theircenter points a distance S with the small nozzles 72 closely packedtherebetween. Thus, the distance between the adjacent nozzle centers isS/2. The centers of adjacent nozzles are offset a distance O. This closepacked arrangement, with small nozzles disposed in the space betweenlarge nozzles, allows for firing in a single pass. Such close packedconfiguration allows gray scale generation, while maintaining highproductivity. The entire composite structure has, for example, 300 dpi(dots per inch) periodicity, but allows a high quality gray scaleprinting that is better than 300×600 and is faster than the true 600×600resolution printing.

[0052] Preferably, for the example of S={fraction (1/300)} inch, thelarge nozzles are at least 40 μm, and preferably 50 μm wide at theirlargest point, and the small nozzles are at least 20 μm, and preferably25 μm at their largest point, with a channel land width between nozzlesof about 5 or 6 μm to achieve adequate sealing. In triangular shapednozzles as shown in FIG. 6, the width would be measured at the base ofthe opening. Large nozzles that are 50 μm wide provide complete spacefilling between spots deposited on the printing substrate with a singlespot size at 300 spi (spots per inch), with low ink viscosity andappropriately sized heating resistors. At 300 spi, the spacing S betweensame size nozzles is about 84.5 μm, with large nozzles at 50 μm andsmall nozzles at 25 μm fit therebetween. By this, the heater centers andchannel centers would be on 600 spi spacing, but in a single printingpass it is possible to use large spots and small spots where desired.This is not possible in prior art arrangements, in which a standard 600spi printhead cannot use channels as large as at least 40-50 μm becausethe channels are on a 42.3 μm centers and require reliable sealing.Thus, to closely pack the different size nozzles, the width of thelarger size nozzle is preferably greater than or equal to S/2.

[0053] Typically, in prior art devices that deposit a single spot size,to ensure overlap of diagonally adjacent spots, the spot size D isselected as S{square root}2 (i.e., 1.414 S) or slightly greater, as seenin FIG. 7A. However, according to the close pack arrangement of thisinvention, the spots do not have to be as large as S{square root}2 tofill the space. Spot sizes of 1.1 S, for the large spots, and 0.8 S, forthe small spots, as shown in FIG. 7B provide complete filling withadditional coverage to allow for misdirected spots. In this case, thearea of the small spots is about half the area of the large spots. Othercombinations of large and small spots are also possible, such as 1.2 Sfor the large spots and 0.6 S for the small spots as shown in FIG. 7C.In this case, the area of the small spots is about one quarter of thearea of the large spots. In each of these arrangements, the printedimage is superior because the small spots protrude less beyond the edgeof the margin of printing. The small spots that do protrude can even betotally or partially eliminated.

[0054] As shown in FIG. 8, the optimal diameter D of the large spot tocompletely cover white spaces with minimum overlap can be determined.Using this determination, an efficient balance can be obtained betweencoverage and ink usage, i.e. the maximum area covered with minimum inkvolume. This is an important parameter in ink deposition due to the inkusage limitations imposed by print cartridge capacity and by requiredink drying time after printing. Ejection of less ink also allows fasterrefill of the channel and enables printing speeds in excess of the speedfor 300×600 spi printing with a single spot size.

[0055] As an example of ink volume savings, referring to FIG. 7A, agrouping of four spots of standard uniform spot size of 1.414 S has atotal area coverage of 2πS². In comparison, FIG. 7B shows a grouping offour spots of diameter 1.1 S and four spots of diameter 0.8 S. In thiscase, the total area coverage is 1.85πS². In FIG. 7C, which shows agrouping of four spots of diameter 1.2 S and four spots of diameter 0.6S, the total area coverage is 1.8πS². This represents a significant inksavings when viewed in the context of a page or entire document ofprint.

[0056] The prior art example of FIG. 7A shows the smallest sized spotthat will completely cover the paper with no white spaces, if all jetsare perfectly directed and all spots have the same size. The examples ofFIG. 7B and FIG. 7C allow greater spot overlap than FIG. 7A andaccordingly allow full coverage even if some spots are slightly small orslightly misdirected. Nevertheless both examples shown in FIGS. 7B and7C use less ink than the prior art FIG. 7A. An even more accuratecomparison of the ink savings can be obtained by comparing the two spotsize arrangement to a single spot size arrangement by calculating theminimum total area of the two spots, which allows full coverage.

[0057] Assuming for purposes of illustration that the diameter of thelarge spots in FIG. 8 is D=aS (where 1.0<a<{square root}2), the point ofintersection of the three adjacent spots occurs a distance x=0.5S(a²−1)^(0.5) from the line joining the two centers. The minimum radiusof the smaller spot is thus r=0.5 S(1−(a²−1)^(0.5)). For perfect overlapof the large and small spots, if the large spot size diameter is 1.2 S,the small spot diameter (2r) must be at least 0.34 S. The area of thefour large spots plus the four small spots isArea=2πS²(a²−(a²−1)^(0.5))=1.553πS². If the large spot diameter is 1.1S, then the small spot diameter must be at least 0.54 S. The total areaof this configuration is 1.503πS². By differentiating the formula forArea with respect to “a” and setting the result to 0, it is found thatthe minimum total area is obtained when the large spot diameter is1.25^(0.5)S=1.12 SV, and the small spot diameter is 0.5 S. The totalarea is then 1.5S². This represents an ink savings of 25% relative tothe single spot size D=1.414 S case in FIG. 7A. In practice, since thelayer of deposited ink is thinner for smaller spots, the drop volumesavings may be even more than 25%.

[0058] Although the above calculation shows the optimal spot sizecombination for minimal ink usage assuming perfect spot placement andperfectly uniform spot size, in actual printing situations there isvariation in both spot placement and spot size. To compensate, it iscommon practice for prior art printheads having a single spot size tomake the spot size a little larger (on the order of 10% larger) than theminimum spot size. For the corresponding optimal spot size combinationfor minimum ink usage in a two-spot-size printhead for actual printingsituations involving misdirection and spot size nonuniformity, thepreferred range of spot diameters is greater than or equal to 1.12 S-5%and less than or equal to 1.12 S+15% for the large spots, and greaterthan or equal to 0.5 S-5% and less than or equal to 0.5 S+20% for thesmall spots. Even here it is understood that a given ink will producedifferent spot size on different papers and that spot size is a functionof temperature in an ink jet printhead.

[0059] Printing with printheads having different size nozzles,especially to achieve gray scale printing, can be accomplished in twopasses with the printhead shifted one pixel between passes so that boththe large and small drops can cover the print grid. As seen in FIGS. 9Aand 9B, in this method, the large and small drops line up on the samegrid. The pixel shift can be accomplished by using two different paperadvance distances, such as a one pixel advance on the left to right passand an N-I pixel advance on the right to left pass, where N is the totalnumber of jets in the printhead.

[0060] Shift can also be accomplished by using a single paper advancedistance if the total number of jets used in divisible by 2, but notdivisible by 4. For example, if the printhead had 128 jets, withalternating large and small channels, only 126 jets would be used. Theadvance distance would then be 63 jet spacings. This allows large and/orsmall spots to be printed at every grid point. The printing throughputpenalty would only be {fraction (2/128)}, which is less than 2%. Theextra pixels could even be used to aid in stitching together theprinthead passes.

[0061] Additional range in gray scale is possible if the small drops areoffset by {fraction (1/2)} pixel from the large drops in the horizontaldirection, as seen in FIGS. 10A and 10B. This can be accomplished byfiring spots according to a staggered firing scheme. By this, the smalldrops can be offset by {fraction (1/2)} pixel by firing all of the largedrops first and then firing the small drops. The jet firing sequence fora 128 jet printhead printing four jets at a time would be 1, 3, 5, 7; 9,11, 13, 15; . . . ; 121, 123, 125, 127; 2, 4, 6, 8; 122, 124, 126, 128.All the large drops will print within half the print cycle time on thenormal drop centers; the small drops will start printing after theprinthead has moved {fraction (1/2)} pixel across the paper. Thus, thedrops will be automatically offset by {fraction (1/2)} pixel in thehorizontal direction, as shown in FIGS. 10A and 10B.

[0062] Another method of printing using staggered firing alternatesbetween groups of large and small nozzles. In this method, banks oflarge (odd) and banks of small (even) pixels are printed alternately,but not the adjacent large and small drops. After the first bank oflarge drops are fired, the small drops half-way down the printhead arefired. The sequence continues, alternating large and small down theprinthead. Each size wraps around to the top of the printhead againafter printing the bottom bank. If the printhead is tilted by 1 pixel,the small drops are offset automatically by {fraction (1/2)} pixel. Inthis case, nozzle openings are aligned along the bar but misaligned, byoffset O, in the perpendicular scan direction because of the differencein heights of the nozzles. The difference in heights of the center ofthe nozzles causes the small drops to be misplaced slightly with respectto the large drops. The difference is in the scan direction, so a slightdelay or advance in the firing of the small jets will compensate for themisalignment and the different size drops will be placed accurately.This staggered firing scheme allows the small pixels to be advancedrelative to the large pixels to compensate for the offset.

[0063] For example, if the nozzle sizes are 25 and 50 microns, thedifference in the heights of the centers is 12 microns (0.0005 inch).For 300 spi printers, if the jets are fired at 6 kHz, the requiredcarriage speed is 20 inches per second. The printhead will cover the 12micron difference in centers in approximately 25 μsec. Thus, if thesmall nozzles are fired 25 μm before or after the large nozzles(depending on the orientation of the printhead and the scan direction),the pixel placement pattern in FIG. 11C representing the standard firingsequence is produced. By this, the large and small pixels can be placedon the same centers without having to fire adjacent pixelssimultaneously.

[0064] To offset the large and small drops by {fraction (1/2)} pixelusing either one of the staggered firing sequences described above, theadjacent small and large jets should be fired 83 μsec ({fraction (1/2)}the print cycle time) plus or minus 25 μsec apart, depending on theorientation of the printhead and the scan direction. The above methodscan be used with any type of printhead that has large and small nozzles,not necessarily those printheads that have alternating large and smallnozzles.

[0065] Image quality can be further improved if the small drops areoffset in the perpendicular direction as well as the scan direction.This increases the ability to print with gray scale and minimize ink forfull coverage. Perpendicular offset can be achieved by tilting theprinthead, which is typically vertically oriented, with respect to thescan direction, as shown in FIG. 11A, with a 45° tilt. Greater tiltangles can be used to increase resolution. Small spots are automaticallyplaced offset by {fraction (1/2)} the spacing of the large drops in bothdirection. A slight staggering of the firing of the large and smallnozzles is necessary to compensate for the offset in height of thenozzles in the scan direction.

[0066] The large nozzle spacing is S on the printhead. When tilted 45°,the printed large spot spacing becomes (S/2){square root}2, seen in FIG.11B. To obtain 300 spi spacing of the large spots on the paper, thelarge nozzles should be centered, for example, on 84.5×1.414=119.5micron spacing, with the small nozzles halfway in between (i.e. achannel to channel center spacing of about 60 microns).

[0067] Another way to achieve perpendicular offset, without tilting, isto displace large and small drops by locating smaller nozzles off-centerwith respect to the channel, as shown in FIG. 12A. For example, for a300 spi printhead, S=84.5 microns, the channel diameter can be 70microns, and the two nozzle sizes can be 20 and 40 microns. If thecenters of the nozzles are both offset as far as possible toward eachother, the spacing is 45 microns. This is approximately {fraction (1/2)}S. By using two passes, advancing the paper an odd number of pixels, andstaggering the printing of the large and small drops, the small dropscan be displaced {fraction (1/2)} pixel in both directions relative tothe large drops, as seen in FIGS. 12B and 12C.

[0068] According to this invention, the printhead 20, having alternatinglarge and small nozzles, can also be operated to print in a single pass.The offset in the printed pixel locations is set by the nozzlelocations, in this case 0.5 S. This offset is provided by ripplingthrough all the odd numbered jets first (the large nozzles), and thenrippling through all the even numbered jets (the small nozzles). Forexample, if 8 jets are fired at a time, the firing sequence in aprinthead having 256 drop ejectors (128 large and 128 small) would be 1,3, 5, 7, 9, 11, 13, 15; . . . ; 241, 243, 245, 247, 249, 251, 253, 255;2, 4, 6, 8, 10, 12, 14, 16; . . . ; 242, 244, 246, 248, 250, 252, 254,256. All the large drops will print within half the print cycle time onthe normal drop centers. Small drops will start printing after theprinthead has moved a half pixel across the paper. This allows completespace filling and gray scale on a single printhead pass.

[0069] Firing the entire set of large drops first then entire set ofsmall drops; allows “tweaking” or adjustment of the small drop position.This is helpful because the line of centers of the taller, largechannels and the line of centers of the shorter, small channels differslightly (by offset O). This offset can be overcome by delaying oradvancing (depending on the scan direction) the firing of the groupingof small channels relative to the firing of the grouping of largechannels.

[0070] This single pass method has the same throughput as printing300×600 spi ROM a similar sized printhead having the same printingfrequency. The difference is a result of the rippling through twodifferent sets of 128 jets, while the 300×600 case will ripple throughthe same set of 128 jets twice in advancing by {fraction (1/300)} inchin the scan direction. A 600×600 printhead having the same printinglength (i.e. 256 jets) and same frequency has only half the printingthroughput because after rippling through all 256 jets it is only ableto advance by {fraction (1/600)} inch in the scan direction.

[0071] Additionally, different pulsing conditions (pulse width and/orvoltage) may be used for the larger and smaller drop ejectors to helpdetermine the size of the ejected droplets. Since only large drops arefired with large drop ejectors (and small with small), different heatersizes with different resistors may be used for the two drop ejectordesigns. The combination of large and small spots provides smoother tonereproduction since halftone cell that uses various combinations of largeand small spot sizes can produce a greater number of gray levels.

[0072] Another printing option is to address each offset grid point onthe printing medium with either large or small spots by using multiplepasses and a printhead advance that successively places rows of smallspots in line with rows of large spots. This method would result in aslower throughput than the above described single pass method.

[0073] While this invention has been described in conjunction withspecific embodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart. For instance, the present invention is not limited to scanning typecarriage printers but also includes partial width scanned printhead,page width type printheads, and full width array abutable printheads.The invention is applicable to monochrome printheads or printheadssegmented to print a variety of colors. Also, while the embodimentsdiscussed have used the example of sideshooter type printheads, theinvention may be extended in obvious ways to the use of roofshooter typeprintheads in which the nozzles may be arranged in two-dimensionalarrays. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

What is claimed is:
 1. A printhead for ejecting droplets of ink to formspots on a printing substrate, comprising: a plurality of drop ejectors,including a first set of drop ejectors having a first size and a secondset of drop ejectors having a second size, wherein the first set of dropejectors and the second set of drop ejectors are arranged in a singlelinear array with adjacent drop ejectors having different sizes to forma pattern of alternating first and second size drop ejectors, whereinthe spots formed by the first size drop ejectors have a diameter D thatequals a product of spacing between same size drop ejectors S andconstant a, according to D=aS (where 1<a<{square root}2), and wherein apoint of intersection between two adjacent first size spots and a secondsize spot occurs a distance x measured from a vertical center lineextending between the adjacent first size spots, according to x=0.5S(a²−1)^(0.5); and an actuator associated with each drop ejector thatselectively actuates the drop ejector to fire ink drops.
 2. Theprinthead of claim 1, wherein the second size spots have a normaldiameter in a range of greater than or equal to 5% less thanS(1−(a²−1)^(0.5)) and less than or equal to 20% more thanS(1−(a²−1)^(0.5)).
 3. The printhead of claim 2, wherein the first sizespots have a nominal diameter in a range of greater than or equal to 5%less than 1.12 S and less than or equal to 15% more than 1.12 S and thenominal diameter of the second size spot is in a range of greater thanor equal to 5% less than 0.5 S and less than or equal to 20% more than0.5 S.
 4. The printhead of claim 1, wherein the drop ejectors include anink channel with a central axis and an end, the end forming a nozzlethat has one of the first and second size, wherein the nozzle is offsetwith respect to the longitudinal axis of the channel.
 5. The printheadof claim 1, wherein the printhead is disposed in a printing deviceincluding a movable carriage that supports the printhead for movement ina scanning direction and a controller connected to the carriage tocontrol movement of the printhead and to the actuators to controlactuator of the drop ejectors.
 6. The printhead of claim 1, wherein thefirst size is larger than the second size.
 7. The printhead of claim 6,wherein the first size is greater than or equal to S/2.
 8. The printheadof claim 1, wherein the first size is greater than or equal to S/2. 9.The printhead of claim 1, wherein each drop ejector in the first set ofdrop ejectors has an axial center point and each drop ejector in thesecond set of drop ejectors has an axial center point, wherein thecenter points of the second set of drop ejectors are diagonally offsetwith respect to the center points of the first set of drop ejectors. 10.A method of firing ink droplets from different size ejectors arranged inan alternating pattern in a linear array on a printhead, including oddnumbered ejectors having a first size and even numbered ejectors havinga second size different from the first size, comprising the steps of:consecutively firing odd numbered ejectors to eject ink spots;consecutively firing even numbered ejectors to eject ink spots; andcontrolling firing of the even numbered ejectors to eject even fired inkspots in spaces between the odd fired ink spots.
 11. The method of claim10, wherein the steps of consecutively firing the odd numbered ejectorsand consecutively firing even numbered ejectors occurs in a singleprinting pass.
 12. The method of claim 10, wherein the step ofconsecutively firing the even numbered ejectors occurs after moving theprinthead a distance equal to {fraction (1/2)} pixel in the scanningdirection.
 13. The method of claim 10, wherein controlling the firing ofthe even numbered ejectors includes delaying or advancing the printheadin the scanning direction relative to a position for firing the oddnumbered ejectors.
 14. The method of claim 10, wherein the step ofconsecutively firing even numbered ejectors occurs in half of the timerequired for one printing cycle.
 15. The method of claim 10, furthercomprising the step of controlling pulsing conditions for each of theeven numbered ejectors and the odd numbered ejectors to control ejecteddroplet size.
 16. The method of claim 10, wherein firing even numberedejectors ejects spots having a diameter smaller than spots ejected fromthe odd numbered ejectors.